Traditional air jet NCTs, first developed in the 1960s, have used a single indentation eye-flattening event (“applanation”) to measure IOP. During an NCT measurement, an air jet generated by a pump mechanism is discharged through a passage in a nosepiece of the NCT at the eye. The air jet creates an increasing pressure on the eye to a level that is adequate to deform the cornea through a first applanated state to a slightly concave state. Subsequently, as the air jet becomes disorganized, the pressure applied to the eye decreases to zero, and the cornea returns through a second applanated state to its original convex shape. IOP is calculated using an internal plenum pressure in the air jet pump mechanism that has a known (predetermined) correlation with the actual pressure exerted on the eye at the moment of inward applanation.
FIGS. 1-3 illustrate a nosepiece 2 and measurement subsystem 1 of a prior art NCT. Nosepiece 2 includes a nosepiece body 3 and a fluid discharge tube 4 held within nosepiece body 3. Fluid discharge tube 4 defines a fluid discharge passage 5 extending from an entry end 6 to an exit end 7 of the nosepiece. Nosepiece 2 is mounted in a measurement head of the NCT at a reference point R. A flange 8 may be provided on nosepiece 2 to facilitate secure mounting. Discharge passage 5 is in flow communication with a fluid pump mechanism of the NCT (not shown).
In preparation for an NCT measurement, the nosepiece 2 of the NCT is aligned with the eye in three dimensions. The fluid pulse discharge passage 5 through the nosepiece defines a fluid pulse axis 20 along which the fluid pulse is directed when it is discharged. The nosepiece 2 is aligned in X (up and down in FIG. 3) and Y (normal to the drawing sheet plane in FIG. 3) directions such that the fluid pulse axis 20 is normal to an apex of the cornea. Additionally, the nosepiece is aligned in a Z direction (left and right in FIG. 3) at a predetermined “working distance” D from the corneal apex defined as the distance along the fluid pulse axis 20 from a fluid exit end of discharge tube 4 (the end arranged flush with the exit end 7 of the nosepiece) to the corneal apex.
In addition to nosepiece 2, subsystem 1 comprises an optical applanation detection apparatus. The applanation detection apparatus includes an emitter 26 arranged and configured to provide a collimated beam along an illumination axis 22 converging with fluid pulse axis 20 at a target point P located a predetermined distance beyond the exit end 7 of nosepiece 2 along the fluid pulse axis. In the arrangement shown in FIG. 3, emitter 26 is an LED surrounded by a sleeve 28 and positioned upstream from an aperture stop tube 30 carrying a window 32. Subsystem 1 further comprises a light-sensitive detector 42 arranged on a detection axis 24 converging with illumination axis 22 and fluid pulse axis 20 at target point P. In the arrangement of FIG. 3, detector 42 is located behind an aperture tube 40, focusing lens 38, aperture stop tube 36, and window 34 all aligned on detection axis 24. The collimated illumination beam obliquely incident to the cornea along illumination axis 22 will be reflected by the corneal surface. When the corneal surface is curved, the collimated illumination beam will be fanned out upon reflection from the curved surface such that only a small portion of the illumination light reaches detector 42. However, when the cornea is applanated to provide a flat reflection surface, the illumination beam will remain collimated and will be reflected along detection axis 24 to reach detector 42 with minimal loss, and the detector 42 will register a sharp peak in intensity corresponding to the applanation event. The applanation detection apparatus described above will be familiar to those skilled in the NCT art. When the NCT is properly aligned for measurement, target point P (intersection of illumination, detection, and fluid pulse axes) coincides with the corneal apex such that the working distance D and the predetermined distance from exit end 8 of nosepiece 2 to target point P are the same distance.
Heretofore, conventional NCTs known to applicant have used a working distance D slightly greater than 11 mm. With proper instrument design, the conventional working distance provides a relative low value of high frequency noise with respect to the increasing pressure applied to the cornea by the fluid pulse. However, it has been discovered that at the conventional working distance of 11 mm, the applied external pressure function becomes quite “noisy” relative to the internal plenum pressure of the pump mechanism as the external pressure applied to the cornea by the fluid pulse decreases. Consequently, accurate correlation between the internal and external pressures is compromised. This has not been a problem for conventional NCTs, which use the initial inward applanation event associated with increasing pulse pressure as the sole basis for determining IOP, and disregard the subsequent outward applanation event associated with decreasing pulse pressure.
However, the problem was recently discovered in connection with a new generation of NCTs that use both the inward applanation event and a subsequent outward applanation event associated with decreasing pulse pressure in determining IOP. These “bi-directional” NCTs were developed by Reichert, Inc., assignee of the present invention, and have been described in U.S. Pat. Nos. 7,481,767; 6,817,981; and 6,419,631. High-frequency noise in the decreasing pressure function is a serious problem for bi-directional NCTs because the corneal-compensated IOP (so-called “IOPcc”) is 2.5 times more sensitive to fluctuations in the second applanation signal than conventional IOP measurements are to fluctuations in the first applanation signal. The second applanation event occurs during the decreasing pressure period.
In principle, this problem could be resolved by simply increasing the conventional air tube diameter (about 2.4 mm) significantly. However, this potential solution creates other problems. These include the need for additional power to drive the pump system, increased vibration and noise due to larger forces generated in the pump, added costs and, most seriously, a significantly increased total force exerted on the eye since the area of application of the pulse pressure would increase. Historically, the major objection to the use of NCTs to measure IOP has been the test subject's neural response to the force of the air jet on the eye during a measurement. The most sensitive nerves responsive to an applied force in the human body are on the surface of the cornea. Therefore, for sake of patient comfort, it is important to avoid an increase in force (pressure times area) on the eye during a measurement as would occur with a larger diameter air tube.
The reason for the “noise” in the decreasing pressure time period is that the fluid pulse is losing collimation and beginning to dissipate where the fluid flow becomes chaotic and unstable. It should be noted that the breakup (noise) of the air jet can be measured with Reichert, Inc.'s tonometer calibration tool described in U.S. Pat. No. 6,679,842. The tonometer calibration tool provides a measurement of the force exerted on a surface located at the working distance of the NCT.